Wikipedia defines flash fiction flashily as
“fiction characterized by its extreme brevity.” When you actually write flash fiction, rather than warnings at the top of encyclopedia articles demanding references for simple definitions, then only certain subgenres of flash are characterized by their extreme brevity. After you’ve written a few stories of exactly 100 words, a loose count of 2,000 is a comparative War and Peace.

In my limited experience, I have found that most flash markets seem to be looking for 500-1000 words, neatly averaging out to the classical definition of 750 words or two pages from the original flash anthology. You can still sell a 2,000 word story, but not necessarily as flash fiction. Oddly enough, 750 words is also the traditional length for a single scene within a larger story, so writing flash is a basic skill no aspiring fantasy trilogist should be without.